It will probably not come as a surprise to many, but the number of acres burned in the United States in 2017 came close to breaking a record. The numbers are preliminary and could change over the next few weeks as the data is finalized but the acres burned in the 50 states, 9,781,062, was the second highest since reliable records have been kept. That is 49 percent higher than the average over the last 10 years. Even as the trend line for the acres burned has increased dramatically since 1990 the total number of fires has generally been slowly decreasing. In 2017, 66,131 fires were reported, which was 4 percent lower than the 10-year average.

But to look at the big picture, at Wildfire Today we like to analyze the national trends without the stats from Alaska, and there are two reasons why. Fires in that huge state are managed far differently from the other 49 states. Most of them are not fully suppressed since they are less likely to endanger people or private property than in the lower 49 states. The second reason is that the fire occurrence is extremely variable, with the acres burned since 1990 ranging, for example, from 43,965 acres in 1995 to 6,645,978 in 2004. Including the Alaska numbers would skew the data for the other 49 states making it more difficult to spot trends.

In case you are wondering why our charts only go back as far as 1990, we are not convinced that the information before that is reliable. In the data provided by the National Interagency Fire Center there was a very sudden, long lasting major shift in the numbers beginning in the early 1980s.

The sloping horizontal lines in the charts represent the statistical linear trend.

A statistic that is quite interesting is the average size. The linear trend line starts at about 22 acres in 1990 and reaches close to 100 acres by 2017. In fact, the average size in 2017 was 139 acres. There could be a number of reasons for this huge increase:

Weather that is warmer and drier making fires more difficult to suppress.

One hundred years of fire suppression has led to forests that are more dense and fires that burn with greater intensity.

A less aggressive strategy is being used on large fires more often for safety reasons.

More fires are allowed to burn naturally without full suppression for environmental concerns.

There may have been a change in the initial attack of new fires, responding with less equipment and personnel.

Another factor to consider is that there was a gradual 30 to 70 percent reduction in the number of large air tankers on exclusive use contracts from 2002 until 2014 when the fleet began to be partially restored.

Above: Map showing the probability of a wildfire of at least 100 acres on January 12 between 1992 and 2015.

(Originally published at 10:40 a.m. MST January 9, 2018)

If you are searching for something to do on a cold winter day the National Weather Service has just what you need — an interactive map system showing the probability of a wildfire of various sizes by date of the year. You can choose what size fire you’re interested in, 100, 300, 1,000, or 5,000 acres and allow the map to animate the entire year, or manually step through in three-day intervals. The data is based on wildfire occurrence between 1992 and 2015.

It is interesting seeing how activity builds and declines in specific areas like southeast Oklahoma, northern Minnesota, and the confluence of Kentucky, West Virginia, and North Carolina.

Thanks go out to Nick Nauslar of the NWS Storm Prediction Center for updating the maps.

Above: Junkins Fire, southwest of Pueblo, Colorado. Photo provided by the Incident Management Team on October 19, 2016.

There have been many discussions recently on this website, in the scientific community, and in the more public arena about why the number of acres burned in wildfires has been increasing rapidly over the last several decades. From the mid-1980s through 2015 the average number of acres burned has grown from about 2 million acres a year to around 8 million. Some people like to claim that this was caused by climate change, environmentalists preventing timber from being harvested, or other factors. However, to complicate the issue, some data appears to indicate that between 1920 and 1950, 10 million to 50 million acres burned each year.

It is very difficult to say that one factor caused fire occurrence to change. While comparing acres burned in the early part of the 20th century to what we are seeing in recent decades, many variables need to be considered:

–Weather and climate trends. This has been vigorously discussed in many venues.

–The capacity to suppress wildfires. In the first two-thirds of the 20th century the ability of land managers to suppress wildfires was very different from what we have today. Fire engines now carry many times more water, and transportation systems enable quicker initial attack response. Helicopters and air tankers were brought into the equation. Heavy equipment became more prolific and capable. More efficient communication and dispatch systems were created. Fires are detected more quickly. Firefighters are routinely brought in from hundreds or even thousands of miles away.

–The wildland-urban interface is growing. More people are living and recreating where previously there was less human activity. This can increase the number of fire starts, and the endangered structures often have an effect on the priorities of firefighters and where they are deployed, as opposed concentrating forces where they are most likely to contain the fire.

—Changes in how timber is managed and harvested.

–Fire suppression can result in longer fire return intervals and increases in the amount of fuel available for the next fire.

–Fuel treatments: mechanical and prescribed fire.

–Changes in the vegetation: non-native species, insects, disease.

–Accuracy of the fire occurrence data. Can the data for 1920 be compared with the data from 2016? The fire occurrence data at NIFC for 1962 through 2015 shows a huge swing beginning in 1983 and 1984, with the number of fires overnight dropping by about 50%, a trend that continued through 2015. This leads one to lose confidence in the data. One would think that the more modern era, post 1984, would have more accurate information than previous decades.

–Changes in wildfire management policy: full, limited, or no suppression.

Our readers will probably suggest even more factors that affect the number of fires and acres burned.

Considering all of these variables, I am skeptical of reports saying that just one is responsible for changes in the number fires and acres burned.

Above: Acres burned in the United States, 1986 through 2015. Data from NIFC, compiled by Bill Gabbert.

A new study released yesterday concludes that human-caused climate change is responsible for nearly doubling the number of acres burned in western United States wildfires during the last 30 years.

Fires in western forests began increasing abruptly in the 1980s, as measured by area burned, the number of large fires, and length of the fire season. The increases have continued, and recently scientists and public officials have in part blamed human-influenced climate change. The new study is perhaps the first to quantify that assertion. “A lot of people are throwing around the words climate change and fire–specifically, last year fire chiefs and the governor of California started calling this the ‘new normal,’ ” said lead author John Abatzoglou, a professor of geography at the University of Idaho. “We wanted to put some numbers on it.”

Warm air can hold more moisture. As the temperature rises the relative humidity decreases. Low humidity withdraws more moisture out of live and dead plants as well as soil. Plants are the fuel for wildfires and lower moisture means fires can burn more rapidly and with increased intensity and resistance to control. Average temperatures in forested parts of the U.S. West have gone up about 2.5 degrees F since 1970, and are expected to keep rising. The resulting drying effect is evident in the rise of more fires.

Jasper Fire pyrocumulus, about two hours after the fire started, August 24, 2000 west of Custer, South Dakota. NPS photo by Bill Gabbert.

The overall increase in fire since the 1980s is about twice what the researchers attribute to climate change; the rest is due to other factors, they say. One has been a long-term natural climate oscillation over the Pacific Ocean that has steered storms away from the western United States. Another: firefighting itself. By constantly putting out fires, authorities have allowed areas they “saved” to build up more dry fuel, which later ignites, causing ever more catastrophic blazes, the researchers say. The costs of fire fighting have risen sharply in step; last year the federal government alone spent more than $2.1 billion. “We’re seeing the consequence of very successful fire suppression, except now it’s not that successful anymore,” said Abatzoglou.

The authors isolated the effects of climate warming from other factors by looking at eight different systems for rating forest aridity; these included the Palmer Drought Severity Index, the MacArthur Forest Fire Danger Index and the Canadian Forest Fire Danger Rating System. They then compared such measurements with observations of actual fires and large-scale climate models that estimate manmade warming. The crunched data showed that 55 percent of the increase in fuel aridity expected to lead to fires could be attributed to human-influenced climate change. Climate’s role in increasing such aridity has grown since 2000, the researchers say, and will continue to do so.

(The graphic below is from the study.)

The researchers found that anthropogenic climate change accounted for about 55% of observed increases in fuel aridity from 1979 to 2015 across western US forests, highlighting both anthropogenic climate change and natural climate variability as important contributors to increased wildfire potential in recent decades.

Mr. Abatzoglou and coauthor Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, say they do not account for some factors that could be offshoots of climate warming, and thus they may be understating the effect. These include millions of trees killed in recent years by beetles that prefer warmer weather, and declines in spring soil moisture brought on by earlier snowmelt. There is also evidence that lighting may increase with warming.

The study does not cover western grasslands. These have seen more fires too, but there is little evidence that climate plays a role there, said Mr. Abatzoglou; rather, the spread of highly flammable invasive grasses appears to be the main driver.

Mike Flannigan, a fire researcher at the University of Alberta, said that previous studies have tried to understand the effects of climate on fires in parts of Canada, but that nothing had been done for the United States on this scale. “What’s great about this paper is that it quantifies this effect, and it does it on a national scale,” he said.

Worldwide, wildfires of all kinds have been increasing, often with a suspected climate connection. Many see a huge fire that leveled part of the northern city of Fort McMurray, Alberta, this May as the result of a warming trend that is drying out northern forests. Fires have even been spreading beyond, into the tundra, in places where blazes have not been seen for thousands of years. That said, fires are not expected to increase everywhere. “Increased fire in a lot of places agrees with the projections,” said Jeremy Littell, a research ecologist with the U.S. Geological Survey in Anchorage, Alaska. “But in many woodlands, the relationship between climate and fire is not as tidy.”

Many scientists studying the issue believe the growth in U.S. western fires will continue for many years. Mr. Williams and others say that eventually, so many western forests will burn, they will become too fragmented for fires to spread easily, and the growth in fire will cease. But, he says, “there’s no hint we’re even getting close to that yet. I’d expect increases to proceed exponentially for at least the next few decades.” In the meantime, he said, “It means getting out of fire’s way. I’d definitely be worried about living in a forested area with only one road in and one road out.”

The number of hectares burned in Canada this year is far ahead of average.

In the United States the acres burned to date are 46 percent above normal. Statistics from May 20, 2016, the last time the National Interagency Fire Center issued a daily Situation Report, show that 1,551,474 acres had burned, compared to the 10-year to-date average of 1,063,835. Almost two-thirds of that was in the Southern Geographical Area — 955,242. The Rocky Mountain Geographical Area also had a surprisingly high figure — 374,846 acres; but that area includes Kansas, where most of the 397,000-acre Anderson Creek Fire occurred in March.

If you disregard that one huge fire, the total in the U.S. is a lot closer to average. But I’m guessing that some politician somewhere is going to take that 46 percent higher than normal figure and run with it.

These numbers do not include prescribed fires. As of May 20 almost 2 million acres in the U.S. have been visited by prescribed fire.